Russia was tonight coming to terms with its most deadly terrorist attack in years after investigators confirmed that a powerful improvised bomb caused Friday's devastating train crash in which at least 26 people, including several top government officials, were killed.

The head of Russia's FSB counter-terrorism agency, Alexander Bortnikov, said the bomb, hidden on the railway line between Moscow and St Petersburg, contained the equivalent of 7kg (15.4lb) of TNT. Officers had found "elements of an explosive device", he said.

Today two huge cranes lifted up wreckage at the crash site as workers searched for the missing. Officials said 18 people were still unaccounted for. Nearly 100 people injured in the crash were being treated in hospitals. Russia's president, Dmitry Medvedev, called for calm amid speculation the explosion could be the start of a new campaign by Chechen extremists. "We need there to be no chaos, because the situation is tense as it is," he told Russian TV.

The luxury Nevsky Express was carrying 682 passengers and 29 crew from Moscow to Russia's second city St Petersburg. It was derailed at 9.34pm on Friday, close to the village of Uglovka, 250 miles north west of Moscow.

Yesterday witnesses described how they heard a "tremendous crash" as the train derailed. "At exactly 9.30pm, 15 minutes after we had passed Bologoye [in the Tver region], we heard an almighty slap," survivor Boris Gruzd told radio station Ekho Moskvy. "It seemed to me as if we had lost a wheel or smashed through some kind of obstacle. I didn't hear any explosion."

Gruzd said the train driver braked severely. The passengers then spent 30 minutes unaware that the last three wagons of the 14-carriage train had flown off the rails. "The first wagon was 1.5-2kms away from the rest of the train. The second had completely flipped over. The third had come off the rails, but was near the main part of the train and was still standing vertically. As far as I know nobody from this wagon was seriously hurt."

Passenger Igor Pechnikov described being in the second of the three derailed cars. "A trembling began, and the carriage jolted violently to the left. I flew through half of the carriage," he said.

Gruzd said that the passengers immediately began collecting warm clothes and mattresses to help the injured. But he said it was extremely difficult to reach people trapped in the mangled carriages – with rescuers peering into the gloom and using flashlights.

So far investigators have not said who they believe planted the homemade bomb. In the days before the crash villagers reported seeing a suspicious individual. "As far as theories go … our main version is that this was an explosion of an unknown device, by unknown individuals. Put simply, it was an act of terror," Vladimir Yakunin, Russia's railways minister, said yesterday.

Yakunin said the incident was "analogous" to another derailment on the same line three years ago, also involving the Nevsky Express, in which 19 people were injured. Russian prosecutors blamed that derailment on Chechen rebels, who have been fighting an on-off war against the Russian state for two decades.

According to Ekho Moskvy, a radical neo-Nazi group opposed to migrants from the former Soviet republics of central Asia has claimed responsibility for Friday's crash, which paralysed train travel yesterday and delayed 27,000 passengers. Other nationalist groups later denied the report.

There seems little doubt that the Kremlin will point the finger of blame at Islamist insurgents currently waging a guerrilla campaign across the north Caucasus. Rebel fighters have carried out numerous attacks in recent months, including suicide bombings, in their apparent attempt to establish an Islamic caliphate.

Russian prosecutors said they believed Pavel Kosolapov, an ex-solider and former associate of the late Chechen rebel leader Shamil Basayev, masterminded the previous derailment. Kosolapov is currently on the run. Prosecutors have arrested two residents of Ingushetia and charged them with helping carry out the 2006 attack.

Yesterday, investigators said they had discovered a 3ft crater beneath the rails where the bomb had gone off. Reuters, however, said that its reporters at the scene had been unable to find it. Earlier, Russian news agencies had quoted transport officials as saying the cause may have been an electrical fault. Russia has a poor record of serious accidents caused by Soviet-era infrastructure.

Among the named dead so far were several senior Kremlin bureaucrats, including Boris Yevstratikov, the head of Russia's Federal Reserve Agency, and Lyudmila Mukhina, a deputy head in the Federal Fishing Agency. A former St Petersburg senator, Sergei Tarasov, also died.